Testing Of A Cost-effective Photovoltaic Thermal Hybrid Solar Collector Prototype Cristina S. Polo López 1,2,*, Luca Tenconi 1, Fabio Lo Castro 1, Stefano Brambilasca 1, Alessandro Virtuani 2 1 IRcCOS S.c.a r.l. Research and certification Institute for Sustainable Construction, Legnano, Milano 20025 (Italy). Phone: +41 58 666 63 14, Fax: +41 58 666 63 49. www.irccos.it 2 Institute of Applied Sustainability to the Built Environment (ISAAC) - Swiss BiPV Competence Centre, University of Applied Sciences and Arts of Southern Switzerland (SUPSI), Cannobio CH-6952 (Switzerland). *Corresponding author: locastro@irccos.com Abstract Today the market is open to new solutions that integrate solar thermal and photovoltaic (PV) devices into a single element in order to generate electricity and heat, the so-called photovoltaic thermal (PV/T or PVT) hybrid solar collectors. The PVT modules improve the performance of PV modules keeping the cells cooled, increasing the overall efficiency of the panel. One stand test for hybrid PVT systems in order to develop new solutions and better knowing their performance under real operating conditions was designed. One innovative low-cost prototype of PVT module was developed using pipes of a floor heating system filled with a special brine to cool the PV cells. To improve module design and to check their behavior in typical domestic hot-water systems, two different PVT solutions, but with identical PV modules, were analyzed without the heat transfer system. This first test assessed modules overall performances. The features of two different collector types are also described. Index Terms solar system, photothermal effects, hybrid power systems, photovoltaic cells, test facilities, thermal variables measurement. I. Introduction A photovoltaic thermal hybrid solar collector, or PVT, hybrid panel collects solar radiation in order to produce simultaneously electricity and thermal energy. The most basic and simplest way to build a PVT panel is coupling the back of a photovoltaic (PV) panel to a heat exchanger. The fluid circulation on the back of the PV panel allows cooling it and the thermal energy removed is then transferred into a thermal storage tank to be used in low temperature thermal applications (e.g. pre-heating domestic hot water, heating systems, increasing the efficiency of a heat pump, etc.). The energy produced by the system is used for the building energy consumption demand. The yield of a PV module depends strongly on the solar cell technology, decreasing with its temperature increase. In a PVT system, the heat exchanger helps reducing the solar cells temperature increasing their efficiency and the output yield of the PV panel. High values of the temperature coefficients for c-si (ca. -0.5%/ C for Pmax) for c-si induce considerable power losses at typical operating temperatures which may become more pronounced for fully integrated installations (BIPV). PVT allows reducing the average operating temp of the modules, consequently reducing the loss of power. Another advantage of using this system is the space optimization: with the same surface of a PV plant is possible to have domestic hot water or waste heat for other applications. This research work is being carried out at IRcCOS, Research and certification Institute for Sustainable Construction, under the Framework Agreement between CNR and the Lombardy Region (Italy), within the WP5 "Testing and development of new technological solutions by using renewable sources". The research project is based on the design and development of an economic, cost-effective, prototype of a new PVT collector after the analysis of different PVT modules available in the market. This paper is a summary of what has been achieved in the first step of the project. II. System description and PVT Prototype specification A. Stand test solutions for testing PVT devices In order to test the new PVT prototype panel and to compare its performance with other solutions, a hydraulic circuit with two different loops was developed between the fifth floor and the terrace of IRcCOS headquarters. The circuit was designed for future development of a third test loop with the possibility to test other solar thermal systems. A real time monitoring system was installed to measure the thermal and electrical performance of the modules. The schematic layout of the test stand facility is shown in Fig. 1 and Fig. 2. An outdoor test facility (Fig, 2) for testing PVT panels under real operating conditions is located on the roof of the IRcCOS building facing SOUTH at 45 tilt.
Fig. 1. Schematic layout of the test stand facility for the parallel testing of two PVTs devices. Fig. 2 Outdoor Stand Test Facility at IRcCOS headquarter in Tecnocity Alto Milanese: a conventional PV module side-by-side the PVT prototype. This test facility allows modifying the tilt and azimuth of the test device and allows the performance evaluation of two different kind of modules in parallel. A pyranometer is mounted on the same plane of the modules to detect the total irradiance. The circuit is equipped with a 300 liters storage tank. Inside the tank there are two heat exchangers made of stainless steel corrugated pipe. The lower coil exchanges heat with the PVT panels while the upper coil is connected to a cooling system. The primary circuit, through actuated valves, is divided into two independent loops that connect to the panels. For both circuits, a fixed speed circulating pump is used. The secondary circuit is connected to a mini-chiller to break down and quickly adjust the inside tank fluid temperature. This is very important both to avoid possible overheating phenomena of the storage tank, both to set precisely the fluid temperature to cool the PVT panel. Each loop of the stand test is equipped with flowmeters for volumetric flow measurement and Pt100 contact and immersion wells resistance thermometers. These sensors indirectly measure the thermal energy being removed from each panel.
We also log the thermal energy stored in the tank by measuring three temperatures at different levels. This way it is possible to simulate a realistic demand of the overall system in a building. Plate heat exchangers are used to transfer heat between the heat storage systems (primary and secondary systems) and the panels and the mini-chiller. The heating fluid flowing through the solar collectors is a glycol-water mixture and water is used for the storage tank. A TMF500 monitoring instrumentation records and monitors the overall system. The electrical measurements are made by a programmable electronic load, MPPT3000 developed at SUPSI (the University of Applied Sciences and Arts of Southern Switzerland), managed with special software to operate every PV panel to its maximum power point tracking and recording data. B. Reliability of the PVT Prototype The PVT thermal energy is obtained removing heat from the PV cells that reach temperatures above 40 C and 50 C (T noct ) during the electricity generation, keeping them cooled. Fluid temperatures that can be achieved with the most efficient systems vary between 30 C and 60 C depending on the PV technology used, on the circuit connection mode and the cooling system, on the ambient temperature and the external irradiation. Whereas values of the T noct vary significantly if it is from an open-rack PV system or mounted to BAPV and to maximum values under BIPV conditions improvements can be important. The major advantage of integrating into a single solution a hybrid thermal PV device, PVT, is primarily due to increased electricity production thanks to cells lower working temperature. This helps to increase the PV cells performance providing less degradation caused by premature aging due to higher working temperatures. The limits of this technology are mainly of technical and commercial nature. The use of a PV module as a conventional solar thermal collector has its limit due to the lack of collectors typical selective materials coatings; consequently the thermal efficiency of these panels is reduced. The commercial limits are mainly due to the higher cost of a PVT panel compared to a traditional PV panel or ST panel. There are additional costs related to the needs of having also a hydraulic circuit for managing the thermal energy produced. Most of the water-cooled PVT panels in the market today are based on the assumption of cooling the PV cells removing heat through a circuit in which the heat transfer fluid circulates. For the cooling system copper pipes or "roll bond " panels are usually used on the back of the PV module; these are custom made expensive materials. These cooling systems may be placed either in direct contact with the Tedlar or welded on a copper or aluminum plate so as to increase the heat exchange surface area. The purpose of this research project was been to manufacture and check the feasibility of a new PTV module self-assembled with cheap materials and components usually available in the building industry, easily adapting also to existing PV plants and different sizes of PV panels. In the experiment electrical and thermal parameters will be monitored in order to verify the effectiveness of a higher PV panel electrical efficiency. The manufacturer of PV panels "Sunerg Solar Energy" has kindly provided two polycrystalline silicon PV panels to be used for the first version of the prototype. The electrical parameters specification of the photovoltaic module is explained in the following table. TABLE I SUMMARY OF TECHNICAL SPECIFICATION OF PV MODULE PV module type XP 36/156-130 Poly_Cristallyn Pm @ STC [Wp] 130,00 Im @ STC [A] 7,43 Vm @ STC [V] 17,50 Isc @ STC [A] 7,80 Voc @ STC [V] 22,00 Fill Factor FF [%] 0,76 Module area [m^2] 1,00 Module Efficiency [%] 13,00 Cell Temperature Coefficient [%/ºC] [Wp] -0,43 [Voc] -0,38 [Isc] 0,10 The photovoltaic module "36/156-130 XP" of 130 Wp used for the experiment contains 36 polycrystalline cells of 156 x 156 mm size and has a length of 1480 mm and width of 680 mm. Fig. 3 shows the technical drawing.. Fig. 3 Technical lay-out of the PV module. III. Experimental Details Today On average commercial c-si PV modules have efficiencies in the range 15-20%. This means that 80-85% of the incident solar energy is lost, reflected, absorbed or converted into heat. We must consider also the fact that the PV cells temperature increase causes a decrease in conversion
efficiency. The solar energy hitting the panel surface will be partly converted into electrical energy and partly converted into heat. For example, a new cost-effective PVT prototype (Fig. 4 and Fig. 5) was developed to check its performance with another commercial PV-hybrid collector. This PVT prototype was designed with low cost materials easily available in the market and easy to handle. The absorber plate was made using dry radiant systems components. These systems are composed by insulated polystyrene pre-shaped panels with 0.5 mm thin galvanized steel slats for heat distribution (figure 5). The steel slats are equipped with a groove to accommodate the piping. The pipes used were multilayers made with a diameter DN10 (14x2 mm) and a thermal conductivity of 0.5 W/mK. are then compared with other PVT panels available in the market today. IV. System Performance Evaluation The first measurements were made on September 20, 2011. Fig. 6, fig. 7 and Fig. 8 show the results that can be obtained using the experimental methodology developed by the authors that allows a complete performance evaluation of different PVT hybrid solar devices under real operating conditions. These figures show the monitored parameters for a typical sunny day. As the charts show, the electrical power production from the new solar PVT prototype closely follows the global radiation curve. Fig. 4 Heat thermal absorbers and polystyrene shaped panel with slat galvanized steel heat diffuser. The panel-slats-tube set was placed on the back of the module in order to collect and transfer the greatest amount of thermal energy to the heat transfer fluid. The polystyrene ensures a high thermal insulation minimizing heat losses. In order to ensure a perfect adherence between the whole elements a plywood has been added. The entire package is then kept under pressure through a tie rods tubular steel system. It is very important that the "cooling" surface keeps in contact to the PV panel so as to maintain always the highest heat exchange surface. Fig. 5 PVT Prototype Package scheme. Two different types of heat absorber were constructed so as to identify the best coils configuration. The new prototype Fig. 6 Electrical power production by the cost-effective PVT module prototype and the same PV module under real environmental conditions. The thermal energy production was also measured. One of the heat storage tanks was used as a calorimeter. It contained 300 liters of water. On the same sunny day in September 2011, the PVT prototype heated the water in the tank from 20ºC up to 24ºC. The heat production was calculated by measuring the temperature difference between the collector outlet and inlet temperature, the fluid massflow and using the glycol-water mixture heat capacity. Under steady-state conditions, the useful heat delivered by the solar collector is equal to the energy absorbed by the heat transfer fluid minus the direct or indirect heat losses from the surface to the surroundings. The useful energy collected from a collector can be obtained from the following formula: q u =A c G t ( ) U L (T p- T a ) = m c p T o -T i (1) E th = q u (2)
Eq. (1) can be modified by substituting inlet fluid temperature (T i ) for the average plate temperature (T p ); if a suitable correction factor is included. The resulting equation is q u = A c F R G t ( ) U L (T i- T a ) (3) where F R is the collector heat removal factor. shown in Fig. 8 the temperature differential begins to be positive from 8:30 until about 19:00 hours and outside this time range there is a loss of energy from the system. Nomenclature Ac total collector aperture area (m 2 ) c p specific heat at constant pressure (J/kg K) E th thermal energy removed (kwh) F R heat removal factor G T total solar irradiance at the collector aperture (W/m 2 ) m mass flow rate of fluid (kg/s) T a average ambient temperature (ºC) T i temperatures of the fluid entering the collector (ºC) T o temperature of the fluid leaving the collector (ºC) T p average temperature of the absorbing surface, (ºC), stagnation temperature (ºC) solar collector heat transfer loss coefficient (W/m 2 ºC) U L Greek symbols transmittance absorptance product Fig. 8 Fluid differential temperature between inlet and outlet, ambient temperature Ta and the cumulative electrical energy delivered on a sunny day in September 2011. Fig. 9 shows the thermal and electrical power output by the PVT module prototype on the same test day, compared to an analogous PV module without cooling circuit. The electrical output increased by a 6.60% and the thermal energy recovered is not neglectable. Fig. 7 Cumulative thermal energy production by the PVT module prototype on a sunny day in September 2011. The first results show that the electric output of this PVhybrid collector is not improved as expected (Fig. 7), but nevertheless a 6% increase in the PV module electrical output was observed. In a cloudy day, the PVT system is in practice useless for electricity production and could lead to a negative overall energy balance because of the water circulation pump power consumption. Another interesting point concerns the fluid temperatures oscillation at the inlet and outlet of the PVT panel. In fact as Fig. 9 Thermal and electrical power output by the PVT module prototype on a sunny day in September 2011, compared to an analogous PV module without cooling circuit. IV. Conclusions Some of the problems linked to the exploitation of solar radiation for energy purposes in the residential sector are the
available surface and their integration into the building envelope. Due to the urban context of the main cities, the climate variability, the buildings predetermined position, the incident solar radiation possibilities and solar spectrum characteristics, solar thermal and photovoltaic systems cannot guarantee all the necessary energy production (thermal or electrical). In order to satisfy the Zero-Energy Building energy requirements it is necessary to use higher surfaces that are not always available. PV-hybrid collector if well architecturally integrated could reduce the visual impact of these elements and solve this problem. The real behavior of this kind of modules and their current performance under real operating conditions are not well known. To study PVT modules and to design and improve new innovative solutions the stand test was developed. This project will allow assessing the problems found during the PVT products tryout in order to improve the test procedures in the future. A complete set of data is not available today, but it is possible to show the first results referred to the start-up period of the experiment with the first PVT prototype. The stand test solution and the data monitoring system have been tested and some minor arrangements have been made to the initial set-up. The results presented in this paper are preliminary results. Meanwhile a newer set of design parameters associated with the development of a novel photovoltaic/thermal PVT solar collector have been examined. The cost-effective prototype proposed here by the authors, using low cost materials from the building industry, demonstrates that PV module increased its electrical performance since the first tests. Further experiments are being conducted to study system performance under various conditions. By re-examining the design of the prototype it will be possible to further increase the system performance. This stand test will be also useful to check the threshold for which it will be convenient the use a specific PVT solution considering both the energy saving by module s thermal energy production and the energy consumption due the circulation pump that decreases the system efficiency. The first test with the low-cost PVT prototype shows that the stand test is a good solution to assess performance and energy production for this kind of modules. Acknowledgement This work was supported by the Framework Agreement between the Lombardy Region and the National Research Council CNR of Italy in the research program: "New technologies and tools for energy efficiency and use of renewable sources in civil end-use". Special thanks to the IRcCOS staff that works in this project. References [1] C. Cariboni, R. Montanari, Solar thermal systems: Advantages in domestic integration, Renewable Energy Vol. 33, 2008, pp. 1364-1373. [2] T.T. 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